The neuroendocrine system has been shown to play a role in the genesis and growth of human and experimental mammary cancer. However, there is still much that remains to be learned regarding endocrine response mechanisms involved in regulating these tumors. One neuroendocrine gland, the pineal, via its hormone melatonin has repeatedly been shown to be inhibitory to carcinogen-induced mammary tumorigenesis, and there are clear examples of its inhibition of human breast cancer cell proliferation. The mechanism(s) by which melatonin inhibits breast cancer growth is not known. However several studies point to a regulatory influence of melatonin on estrogen receptor expression. With the availability of cDNA probes and monoclonal antibodies to the human estrogen receptor, as well as to genes and proteins which are estrogen regulated, a comprehensive project is now proposed that will examine the growth parameters and estrogen receptor expression of breast cancer cells in response to melatonin, and will correlate these results with the expression of known estrogen regulated growth factors. Five studies are proposed which are designed to answer the following questions regarding the role of melatonin in the growth and physiology of human breast cancer: (1) Are the growth inhibitory effects of melatonin on human breast cancer mediated thru the cell's own endocrine response system and/or thru serum growth factors? (2) Does melatonin alter estrogen receptor expression and, if so, at what level(s)? (3) Does alteration of estrogen receptor expression by melatonin result in changes in estrogen regulated proteins and growth factors? (4) Does melatonin inhibit growth of breast cancer cells via mechanisms which are not estrogen regulated? (5) Does the in vivo potency of melatonin compare with its in vitro action, and at what level does inhibition of mammary tumor cell growth occur in situ? This project will provide the answers to these questions and will ultimately determine the biological and functional significance of the pineal gland and its hormone melatonin in altering the growth of estrogen-responsive breast cancer.